Electrostatic loudspeaker

ABSTRACT

An electrostatic loud speaker system is shown which combines a balanced transducer, an amplifier and an enclosure each of unique construction which together permit the reproduction of frequencies over the full audio range. The electrostatic transducer is shown surrounded by an enclosure that has an outlet passage preferably significantly smaller than the transducer and an acoustic lens preferably guides the sound through the narrow outlet into a wave form of circular cross-section. By these provisions a low resonant frequency for the speaker and wide dispersal of the directional high frequencies are achieved in an enclosure of limited size. The fixed electrodes of the transducer are of substantial thickness and are formed of high dielectric constant material, achieved preferably by molding a lower K matrix with additives raising K and lowering volumetric resistivity. The amplifier is formed of series-connected active devices, one controlled by the other. A third active device amplifies the audio signal. Its output is connected to control the first of the series-connected devices and the output terminal of the amplifier is connected through a resistive feedback path to the output of the third device. A further feedback system employs a carrier wave applied to the diaphragm of the transducer. The resulting signal on the electrodes is differentiated and negatively fed back to damp speaker response at low frequency resonance.

United States Patent Beveridge 1 June 6, 1972 [54] ELECTROSTATICLOUDSPEAKER Harold N. Beverldge, 1616 Franceschi Road, Santa Barbara,Calif. 93130 [22] Filed: June 17, 1969 [21] Appl.No.: 833,952

[72] Inventor:

[52] US. Cl. ..l79/1l1 R [51 Int. Cl ..I'I04r 19/02 [58] FleldofSearch.179/11 l-l 15.511;

[56] References Cited FOREIGN PATENTS OR APPLICATIONS 1,123,333 6/1956France ..179/1ll OTHER'PUBLICATIONS Jordan; Loudspeakers, Focal Press,1963 pp. 122- 123, 170- PrimaryExaminer-Kathleen H. Claffy AssistantExaminerlhomas L. Kundert Attomey-John Noel Williams VOLUME I [57]ABSTRACT An electrostatic loud speaker system is shown which combines abalanced transducer, an amplifier and an enclosure each of uniqueconstruction which together permit the reproduction of frequencies overthe full audio range. The electrostatic transducer is shown surroundedby an enclosure that has an outlet passage preferably significantlysmaller than the transducer and an acoustic lens preferably guides thesound through the narrow outlet into a wave form of circularcrosssection. By these provisions a low resonant frequency for thespeaker and wide dispersal of the directional high frequencies areachieved in an enclosure of limited size. The fixed electrodes of thetransducer are of substantial thickness and are formed of highdielectric constant material, achieved preferably by molding a lower Kmatrix with additives raising K and lowering volumetric resistivity. Theamplifier is formed of series-connected active devices, one controlledby the other. A third active device amplifies the audio signal. Its outiput is connected to control the first of the series-connected devicesand the output terminal of the amplifier is connected through aresistive feedback path to the output of the third device. A furtherfeedback system employs a carrier wave applied to the diaphragm of thetransducer. The resulting signal on the electrodes is differentiated andnegatively fed back to damp speaker response at low frequency resonance.

22 Claims, 23 Drawing Figures VOLUME II FEEDBACK 3 p dis -44 3'4 INVERTEFIG I VOLUME I PATENTEDJUH 6I972 3,668,335

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PATENTEnJun 6 I972 SHEET 08 [IF 11 FIG.

IOBIWITH db OF FEEDBACK APPLIED) I06 DIFFERENTATED DIAPHRAGM EXCURSIONSIGNAL) 200 400 600 8OOIOOO FIG. I4

PATENTEDJun 6|972 sum 10 0F 11 PATENTEUJUR 6 I972 SHEET norm W SQQN Q89i W Z W H w QOGSW DI \N\ LI v a A QM QwmQN l high frequencies.

ELECTROSTATIC LOUDSPEAKER This invention relates to electrostaticloudspeakers. A principal object of the invention is to provide anelectrostatic speakersystem and components which solve or avoid in apractical way. the yarious problemswhichfexist for such speakers.

, Another object is to provide an electrostatic loudspeaker capable ofaccurately reproducing sounds down to 40 to. 50 Hz U .withamplevolumeandwhich is sufficiently compact as to be acceptablefor use. in homes.

(Another-object isto provide an electrostaticloudspeaker capable ofaccurately reproducing high frequency. sound, frequencies from 1,000 toabout 10,000 Hz and dispersing it over a wide solid angle (e.g. anglesup to 180).

,Ano ther object is to provide a full range electrostatic loudspeakercapable of accurate reproduction of both low and .Another object is toprovidean electrostatic loudspeaker hayingan electrode construction andpower source capable of achieving each and all of the foregoing objectsas well as providing better performance generally for electrostaticspeakers.

' The invention features the combination of an electrostatic transducerwith particular components to achieve the above objects. Featured is anelectrostatic transducer surrounded by a rigid enclosure andhaving aconstricted outlet throatsmaller .than. the transducer, using thevprinciple of the Helmholtz resonator, enabling the achievement of anacceptablylow figure (e.g. below 50 Hz) for the natural frequency ofresonance of the mechanical system in an enclosure which is sufficientlysmall to enable use in the home. Preferably, the volume of the enclosureexposed to the back side of the diaphragm is at least four times greaterthan the.volume.exposed to the front.

The invention further features an electrostatic transducer with meansestablishing equal length paths from various diaphragm portionsterminating on the arc of a circle having a center on the sound pathsome distance from the diaphragm shortest wavelength sound tobedispersed, generally less than l k inches and on the order oftwo-third inch for speakers having high frequency capability.

The invention further features a high voltage power system, with peakvoltages exceeding 3,000 volts, employing thick electrodes of highdielectric constant (e.g. K 60) and a relatively low volume resistivity(e.g. R, within the range of IO" and 10'', preferably on the order of10' ohm centimeters.

Thick electrodes according to the invention may be con- :Jdered to bethose whose dielectric portions are more than about 3 times that of theair gap involved. While large air gaps, electrode thicknesses on theorder of one-fourth inch, and peak voltages in the range of'6,000 voltsare employed for the preferred full range speaker, in its broader aspectthis thick electrode feature of the invention applies to constructionsinwhich both the air gap and the dielectric dimensions are scaled tosmaller'values. Such thick electrode systems avoid power arcs whileachieving a relatively high energy output per unit diaphragm area,judged against other electrostatic speakers. The electrodes concentratethe electrostatic field in the air gap between the electrodes, protectthe conductive diaphragm from arcing and enable voltage differencesacross the thickness of the electrodes to be rapidly eliminated, shouldsuch voltage differences occur. The transducer can present a capacitanceon the order of 750 to 1,000 pf per square foot of diaphragm area andtotal capacitance ranging up to 2,000 pf it and above, with virtually noresistive impedance in the operativefrequencyrange, thus providing amplecapability for the full range speaker of the invention.

To provide the high K electrodes the invention features an electrodemolded of a dielectrici'natrix material in which is dispersed asubstance of relatively much higher dielectric constant, e.g. K 500, toachieve an electrode K advantageously greater than 30. A furtheradditive is useful to lower the volume resistivity of the electrode topreferably on the order of IO ohm centimeters to achieve appropriatetime constants.

Also featured is an amplifier capable of powering this system, theamplifier having active devices series-connected across alargeDC-voltage, with the output terminal for the electrostatic transducerlocated between the active devices. A

thirdactivedevicehaving high impedance (constant current .generator)characteristics is controlled by the audio signal source and its outputis connected to control one of the active devices, while itis alsoconnected through a resistive feedback path to the output terminalflhisachieves a high-voltage, lowimpedance amplifier directly coupled to theelectrostatic transducer.

Further featured'is an electronic feedback loop which dampens theresponse-of the speaker in the low frequency range. This feedback loopemploys a carrier signal applied to the moving diaphragm, and detectorcircuits for both electrodes of a balanced speaker, the dielectricportion of the electrodes having a high K, e.g. K l 5, at the carrierfrequency. Demodu- .lation recovers a signal that varies withdisplacement of the diaphragm and differentiation of the signal providesan effective feedback signal proportional to diaphragm velocity and ofproper phase.

system are capable of better sound reproduction in the home than any:fullrange speaker system heretofore proposed; and

.40. portions, the speaker thus being capable of dispersing high I thatother versions permit a good level of quality to be achievedinexpensively.

While'these features and others which will appear herein lead to a fullfrequency range electrostatic loudspeaker, they have-use individuallyand in various subcombinations, as will be apparent to those skilled inthe art.

.In-the drawings: FIG. 1' is-a horizontal cross-sectional view of a fullaudio 'rangeelectrostatic speaker according to the invention,includsystem of FIGS. 1 and la, showing the outlet;

FIG. 5 is a perspective view similar to FIG. 4 viewed from the back toreveal the inlet of the lens system;

FIG. 6 is a cross-sectional view of an alternate form of lensconstruction;

FIG. 6a is a horizontal cross-sectional view of a transducer for use inthe embodiment of FIG. 6;

FIG. 7" is a partially broken away perspective view of a preferredelectrode plate; 1

FIGS. 7a and 7b are plots of electrical characteristics of electrodematerial according to the invention;

FIG. 8 is a cross-sectional view showing an edge member being formed onthe electrode plate of FIG. 7;

FIG. 9 is a cross-sectional view of a balanced electrostatic transducerbeing formed from two electrodes in accordance with FIG. 8;

FIG. 10 is a schematic utilized to illustrate the difficulty of usingconventional amplifier circuitry to drive a capacitive load;

FIG. 11 is a simplified schematic of an amplifier of the type providedby the present invention;

FIG. 12 is a block diagram of an entire loudspeaker system according tothe invention; and

FIG. 13 is a detailed schematic of the electronic components of onepractical embodiment of the invention.

FIG. 14 and 15 are plots of characteristics of a preferred embodiment ofthe invention.

FIGS. 16, 17 and 18 together comprise a detailed schematic of anothercircuit according to the invention.

Referring to FIGS. l-3 there is shown an embodiment of a full rangeelectrostatic loudspeaker in accordance with the invention. The basiccomponents comprise an electrostatic transducer 10 (including a largeflexible diaphragm 12 eg of metal coated mylar, and a pair of rigidplanar high K electrodes l4, 16), a rigid-walled enclosure 18surrounding the transducer 10, an outlet passage, here in the form of alens 20 and an amplifier 22 including a diaphragm-tracking feedbackcircuit 23.

The electrostatic transducer 10 extends across one third of the fullwidth W of the enclosure 18, having a width W,, of 13 inches. Theelectrode assembly of the transducer has a height of 23 inches. A numberof these can be mounted above each other if desired.

The electrostatic transducer of this embodiment is of the balanced typein which the flexible diaphragm 12 is held in taut condition between twoapertured electrodes 14, 16. The sound absorbent material 19 (effectivedown to about 300 H and the rigid walled enclosure 18 prevent backwardmoving radiation emitted by diaphragm 12 back through electrode 14 fromescaping and causing cancellation of the forward radiatron.

The forward electrode 16 is disposed immediately adjacent the inlet 20,of the lens structure 20 (see FIG. 5). The lens is composed of a seriesof walls 20,, 20 ".20 (see FIG. 1a) which are straight in the verticaldirection (see FIGS. 4 and 5) but are spaced apart and curved inaccordance with a special pattern in the horizontal direction to definea series of channels (see FIGS. 1 and 1a). Thus outer wall 20 and thenext adjacent wall 20 define a channel (channel I) having an inlet ofwidth W, exposed to a corresponding outer portion of diaphragm 12(through the apertures 16a of the outer electrode 16). The walls 20, and20 converge together in the direction outwardly and simultaneously curvetoward the centerline of the lens, to the lends throat region 20,.

Near this region the channels begin a re-entrant curve so that at thethroat 20, the channel is again substantially perpendicular to thediaphragm, although displaced significantly toward the centerline.Beyond this region the walls 20, and 20 curve outwardly from thecenterline and diverge from each other, terminating in ends 20e which,in this example, are disposed outside of the front wall 18a of enclosure18. The axis A, of the outlet of channel 1 is thus directed outwardly ata substantial angle from its direction of the channel axis at the inlet.In like manner the other side of wall 20 and wall 20 define channel II.It is disposed to receive the sonic output of the next adjacent portionof the diaphragm. It curves and converges and diverges similarly tochannel I while its output axis A is disposed at a lesser angle to thenormal to the diaphragm. Channel II provides the next adjacent segmentof the solid angle a achieved by the lens. Channel III is likewisedefined by the walls 20 and 20,, and so on to Channel IX, along whichextends the centerline. The lens structure is symmetrical about thecenterline, and thus the right hand outer channel XVIII curves in likemanner, but in opposite direction, to Channel I.

The outer portion of the walls 20, 20,,, are shaped to establish theseries of outlet axes A, A,,.,, such that projections of these axesintersect at a common inward point C spaced substantially (eg 1 foot)from the diaphragm. Since a dispersed angle a of about one half a circleis desired for this embodiment center C lies on the plane projectedthrough the front surface 18a of the enclosure. Preferably, as shown,the

curvatures of the walls are arranged so that the sound path P along eachof the channels and outwardly to a circle projected from the commoncenter C of the outlets is the same length for all channels.

The effect of these features is to emit a circular wave front eventhough the sound emitting diaphragm is both planar and extremelydirectional for the high frequencies. With a suitable shaping of thewalls, the wave form can be spherical, however in the preferredembodiment shown, the speaker retains the same circular horizontalcross-section throughout its height, hence the output sound wave is ofcylindrical form, which can spread to fill a room with high frequencysound. The walls may be made of various conventional speaker materials,e. g. paper stock of appropriate grade. The outer channels may be oflesser width than the inner channels (e.g. W, W taking advantage of thefact that the smaller the filament of sound, the more it can be bentwithout distortion. For outer channels especially, the channel widthshould be based upon the shortest audio wave length of interest and ingeneral should be less than 1 A inches. Practical limits exist howeverbecause too narrow a channel introduces too much resistance to thetravel of the sound. Thus it is found that channel width on the order oftwo-thirds of an inch for the channels is suitable. A practical rule,for channels which turn significantly, is that the inlet width of thechannel should approximate the wave length of the highest frequency ofinterest.

The potential mid frequency mismatch resulting from reflected waves ofthe channels is believed to be overcome by varying from channel tochannel the distance from the diaphragm to the point (20,) at which theconstricted throat occurs. Referring still to FIGS. 1 and In it will beobserved that although the throat 20, for the various channels occurs atthe same plane parallel to the diaphragm, this represents different pathlengths to the diaphragm. Thus the path length of channel I from itsportion of the diaphragm to the throat 20, is longer than thecorresponding path of the next adjacent channel II.

The throat 20, is sized, in relation to the given enclosure, to providea sufficiently low natural frequency of resonance (e.g. below thefrequencies of bass organ and bass drum) in accordance with the lawsrelating to Helmholtz resonators, and in general will be less than halfas wide as the transducer for enclosures of suitably limited size.

Also the volume of the enclosure exposed to the back side of thetransducer (Volume I, FIG. 1) should be a plurality of times greaterthan the volume of the enclosure exposed to the front side of thetransducer, as measured from the diaphragm 12 to the outlet (Volume II,FIG. 1). Advantageously the ratio of Volume I to Volume II should begreater than 4.

In this embodiment the restricted throat is 3 inches wide contrasted tothe l-foot width of the sound-emitting diaphragm which is surrounded bythis enclosure, and the lens outlet has a width of about 7 inches. Theenclosure width W is 36 inches, depth d is l8 inches and height h is 72inches, the front edges of the enclosure being chamfered as shown.Volume I bears the ratio to volume II of about 10 to 1.

In this embodiment the ends 20e of the walls defining the channelsprotrude beyond front wall 18a and the axes A, and A of the outlets ofchannels I and XVIII on the extremities are substantially parallel tofront wall 180, see FIG. 1a, in order to achieve a solid angle ofdispersion or approximately The ends 20e are hidden from view in asimple manner by grill cloth 17. In this case the cloth is anchored atpoints 21 and 22 at the beginning of the chamfer of the front edges ofthe enclosure (the chamfers decrease the bulky appearance of theenclosure). The grill cloth extends at a substantially similar angle tothe chamfers to stand-off projections 23 located at the intersection ofthe chambers and the front panel 18a, these projections beingacoustically transparent. The projections 23 extend as far from thefront panel 18a as do the ends 20e of the channel-forming walls, and thegrill cloth stretched between these projections covers these ends andgives the speaker a finished appearance.

cies (e.g. bass organ and bass It may be noted that in the instance ofusing such a speaker as this merely as a woofer, only the outside. wallsand 20 of the lens,-FIG. la, would be required, still keeping theconstricted throat. However, high and low frequency component mismatch,cross-over network difficulties and other significant problems areavoided and economies achieved by the vfull range speaker of the presentembodiment.

To illustrate that the concept for reducing the frequency of lowfrequency resonance and dispersal of high frequencies by a restrictedaperture lens system may not be limited to the preferred channeled'lensconstruction, reference is made to FIGS. 6 and 6a. There is shown anenclosure surrounding an electrostatic driver 32. The principle of thisspeaker would be the same as that of the preceding figures in thisrespect: a

restricted aperture 31, or throat which is substantially narrower thanthe surrounded driver 32 ple of the Helmholtz resonator) resonance ofthe mechanical system provides (by the princia natural frequency ofbelow the lowest frequendrum) that are to be reproduced, and thebackward radiation of the transducer is contained by the enclosure. Atthe same time the transducer is arranged so that its sounds seem toemanate from a center C spaced from the diaphragm and near the frontwall 30a of the enclosure. The paths P from adjacent portions of thediaphragm (32,, 32 32 radiate from this center so as to disperse thehigh frequency sounds through a wide solid angle.

The lengths of the paths (e. g. p,, p p,,) from the respective portionsof the diaphragm to a circle centered on C are substantially of the samelength.

Referring to.the specific structure of FIGS. 6 and 6a, the form of thetransducer 32 approximates the arc of a circle and the concave portionof the transducer is directed toward the aperture 31. (Although a singlecircular line is shown in FIG. 6, it will be understood that thetransducer preferably com prises opposed apertured electrodes with aflexible diaphragm disposed therebetween in a balanced construction.)The transducer is arranged with the focal point (the point ofintersection of the normals to the'diaphragm surface) at the position ofthe desired center for radiation C, e.'g. near the plane 30a of thefront of the enclosure. Thusthe high frequency radiations,

in following paths normal to the diaphragm portions, pass through thecenter, adjacentpaths crossing each other. Since the normals of adjacentsurface portions lie at angles to each other;.the net effect is acircular wave form emanating from the aperture. FIG. 6 is a horizontalcross-section of the speaker. The speaker can have uniform cross-sectionthroughout a substantial height, so it generates a cylindrical waveform, to disperse high frequency radiation in a wide solid angle a.

As illustrated in FIG. 6a, it may be possible to. approximate a circularcross-section transducer by a number of planar units 32,, 32 32,,disposed as chords of a circle, thus to take advantage of thesimplifications attendant with the use of planar members. It also may beadvantageous to employ rounded transition surfaces 35 between the frontwall 30a of the enclosure and the guide walls 36 which extend from eachouter diaphragm portion to the aperture 31. Thislens construction couldbe used in tweeter and mid-range speakers as can the lens system of FIG.l3, but one of its virtues, like that of the system of FIG. l3, would bethe possibility of use in lower range and full frequency range speakersin which a lowered resonant frequency and avoidance of cancellation andreinforcement due to the back wave would be achieved.

Referring to FIG. 9, in the preferred embodiment of the invention aconductive diaphragm 12 is employed. With such diaphragms the use of abare conductive fixed electrode would lead to power arcs that candestroy the diaphragm. It is realized thatcoating of the electrodes withhigh dielectric strength coatings would still leave the problem ofimperfection and pinholes permitting destructive power arcs. This wholeclass of problem is avoided according to the invention by using thickhigh dielectric constant electrodes, i.e. dielectric portions about 3 ormore times greater in thickness than the air gap on the respective sideof the diaphragm, to be in mid-position.

assuming the diaphragm The electrode featured by the invention formeeting these unusual dielectric requirements is a molded membercomprised of a dielectric matrix in which a substance of relativelyhigher dielectric constant (preferably K 500) has been dispersed. Theresult is an electrode whose construction permits tailor-made electricalproperties with high K values. Such electrodes permit sufficient forceper unit of diaphragm area to be applied to power the loudspeaker beingdescribed.

The matrix material may be selected from the various moldable dielectricsubstances that are available, but particularly good results areachieved using epoxy, which has dielectric constants below 10, e.g. K 4to 6. To this material is added, previous to molding, a dispersion of asubstance selected for having a much higher dielectric constant (such asbarium titanate, K 1,000 tol,500). Advantageously, also a dispersion ofsemi-conductive substance (such as carbon) is added, having a lowervolume resistivity than the matrix material to achieve a volumeresistivity in the range of 10'' to 10" ohm centimeters, the preferredvalue being about l0 ohm centimeters. I

The properties of the resulting electrode are not linear. That is,whereas a linear resistor has a slope of one in the IE plot, for theactual material the slope is on the order of one-third. Also thedielectric constant at high frequency drops to approximately two-thirdsof its low frequency value. Some idea of the operational properties ofthe material may be seen by computing a pseudo time constant formid-range values, with the assumption of linearity of these properties.Referring to FIGS. 7a and 7b, curves of a typical embodiment, thematerial is seen at midfrequency range to have a dielectric constant ofabout 40 and a volume resistivity of 2 X 10 ohm centimeters. Thiscorresponds to a pseudo time constant of 0.2 seconds. In actuality thetime constant at this particular frequency will be somewhat longer dueto the lesser slope of the volume resistivity curve of the actualmaterial.

It is found that time constants considerably greater than I second makerecovery of operation of the, speaker in the event of overload undulylong. On'the other hand, if volume resistivity is too low, in the caseof the diaphragm touching the electrode, too large a current flows whichcan impair the tension of the mylar diaphragm.

With the dielectric constant K in.the preferred range of 30 to 60 andwith the volume resistivity lying within the range of about 10 and 10"ohm centimeters, satisfactory performance is'obtained.

The dielectric constant of the material according to this inventionremains substantially high into the range of several hundred kilocyclespermitting theuse of a carrier frequency for measuring diaphragmmovement and velocity for negative feedback purposes, discussed furtherbelow.

According to asuitable procedure for preparing the electrode bariumtitanate and carbon powders are mixed together with a suitablyproportioned mass of epoxy in the liquid state, prior to reaction. Themixture is then cast into a mold in the desired form and cured.

Referring to FIG. 7 a broken away portion of the preferred electrodeplate is shown, formed by the casting procedure. The matrix materialillustrated by the dashed cross-hatching is No. 2038- epoxy; and No.3416 hardener, manufactured by I-Ioughton Laboratories of Clean, NewYork, in the ratio of 1 part by weight hardener to 10 parts 2038.

The triangles shown in the cross-section of FIG. 7 diagrammaticallysuggest the uniform dispersion of fine barium titanate particles and thecircles shown in the cross-section similarly suggest the'uniformdispersion of carbon particles. Employing substantially equal amounts ofbarium titanate and 2038, with carbon approximately 5 percent of theweight of the epoxy produces a suitable electrode construction withdielectric constant in the range of K= 30 to 40 and volume resistivityin the region of R,. 10" ohm centimeters.

In one preferred embodiment the electrode plate consists of thefollowing percentages, of the various ingredients:

epoxy 2038 I00 parts by weight "HA-n 10 parts by weight 5.3 parts byweight (being 4.8 percent of the total epoxy) epoxy hardener 3416 carbonReferring again to FIG. 7, the slots molded integrally into theelectrode plate 14 have length L on the order of 2 inches, inlet width Sof 1 H16 inch, and the slot walls diverge to an outlet width S ofone-eighth inch. Each land between the slots has a width on the inletside of three-sixteenth inch and converges to a width of one-eighth inchon the outlet side. The electrode thickness 1 is on the order ofone-fourth inch.

The inlet surface of the electrode is cast precisely planar and smooth.The outlet surface is smooth and after formation is coated with aconductive layer 140, FIG. 9, e.g. an epoxy containing a dispersion offine silver-coated particles. Thereafter, the electrode plate may bebaked for curing, e.g. 140 F. for a few hours.

After formation the electrode plate 140 is appropriately jigged, seeFIG. 8, and an edge member 14b ismolded integrally therewith. A planarcasting plate 38, e.g. of plate glass forms the inside edge surface 140of the edge member 14b. A removable spacer 40 of uniform thicknessapproximating onehalf the thickness of the desired air gap rests uponthe casting plate 38 and directly supports the inlet surface of theelectrode during this operation. Jig members 42 and 44 form the outlineof the edge member 14b. The edge member may be formed of the samematerial as the matrix of the electrode plate, however omitting theadditives.

Referring to FIG. 9, two such electrode members 14 and 16 are broughttogether, inner surfaces directed toward each other and frame surfaces14c aligned. A thin flexible conductive diaphragm 12 (e.g. a polyesterfilm such as Duponts Mylar, of between one-fourth to one-half milthickness, carrying on each side a vacuum-deposited aluminum coating) isdisposed between the electrodes. Tension T (of several thousand p.s.i.)is applied to the diaphragm beyond the electrode whereupon theelectrodes are permanently clamped to the diaphragms, e.g. by means ofadhesive applied to mating surfaces Me or by bolting the two electrodestogether. The thus formed electrostatic driver is then ready formounting within the speaker enclosure, see FIGS. 1-3.

For full range electrostatic speakers the polarizing voltage across thefixed electrodes may range between about 2 to 8 kilovolts. The air gapbetween the diaphragm (in midposition) and either fixed electrode rangesbetween one-twentieth to one-tenth inch and it is found that thethickness of the electrode preferably should be of the order ofone-fourth inch.

The signal voltage is divided across the electrode thickness and the airgap. To lose as little signal voltage across the electrode and toconcentrate the signal voltage to the air gap the electrode of highdielectric constant (e.g. greater than 30) is employed. A maximum limiton electrode thickness is found to exist because of lowering of the slotresonant frequency with increasing thickness. A minimum thickness limitis found to exist because of the need to avoid power arcs. For higher K,lower conductance is needed, but R, too low is found to give problems,for example greater tendency for the diaphragm to stick to the electrodeshould contact be made, and localized heating and resultant damage tothe diaphragm.

The desired level of leakiness (i.e. (i.e. the volume resistivity) isadvantageously achieved by the addition of a dispersion of carbon, asnoted above.

In the example of the preferred embodiment, see FIG. 9, the polarizingvoltage together with the audio peak is established at 6,000 voltsbetween diaphragm and electrode, the air gap A, is 0.070 inch, theeffective dielectric constant of the electrode material averages K 40(varying little with frequency), the thickness t of the electrode isabout one-fourth inch. The volume resistivity of the electrode isadjusted by the amount of carbon present in the electrode matrix,(between 10" and 10 ohm centimeters) to establish a time constant ofless than 1 second, preferably less than 0.1 second. By this is meantthat less than 1 or 0.1 second is required for a voltage between theinner and outer faces of the electrode to drop to one-third of itsvalue.

The electrostatic speaker of the invention imposes severe operatingrequirements upon the associated amplifier system. To obtain therequisite levels of audio output without requiring an unduly largediaphragm, the audio drive voltage must be quite high, a peak drivepotential of 4,000 volts being employed. The speaker impedance is almostentirely capacitive (3,000 to 6,000 pf.) and peak currents in excess of300 ma. are sometimes required implying a peak output requirement ofmany hundreds of bolt amperes. The output transformers generally used todrive conventional moving-coil speakers (which have a relatively low andessentially resistive impedance, e.g. 8 ohms) are peculiarly ill suitedto the demands of the electrostatic speaker. Transformers having therequisite output are cumbersome, expensive, and present resonanceproblems when used to drive a capacitive load.

A simple schematic is presented in FIG. 10 to illustrate thedifficulties of driving an electrostatic speaker directly from theoutput of a conventional resistance-coupled amplifier. An audio inputsignal 52 is applied to amplifying tube 54 and a plate supply voltage of4,000 volts is applied to terminal 56. If capacitive load 60 is 3,000pf. its impedance at 10 KHz is about 5,500 ohms. For the loss inresponse at that frequency to be limited to 3 db. plate resistor 58 canbe no larger than 5,500 ohms. The plate supply would then have tofurnish about 375 ma. or 1,500 watts to terminal 56. This "brute force"method of driving a speaker is, of course, highly inefficient andimpractical.

The inventor has devised a low-impedance amplifier circuit that producesthe required output with efficiency, stability, linearity, and arelatively low-level input. A simplified schematic of this circuit isshown in FIG. 11. An audio input signal 52 of about lO-volt amplitude isapplied to the grid of pentode T3. (Like elements are designated withidentical reference numerals throughout all the figures.) The terminal56 plate supply of pentode T2 is +4,000 volts. Capacitor 60,representing the load presented by the electrostatic speaker, has avalue of 3,000 pf. A volt potential is applied to 10 K cathode resistor62 at terminal 64. The output signal at point B ranges from a valueclose to ground to almost +4,000 volts, and may furnish a peak currentof over 300 ma. in either direction to load 60.

In the quiescent no-input state, pentode T3 develops a well definedplate current of slightly over 1 ma. through the 2M feedback resistor66. About 2,040 volts is developed over resistor 66 so that point Bsettles at a quiescent voltage of about +2,000 volts, while point Asettles at about 40 volts. About 6 ma. flows from terminal 56 throughpentode T2, 7 K resistor 68 and pentode T1.

Point B is held stable at +2,000 volts by feedback resistor 66. Shouldit tend to rise to a higher voltage, the voltage increase would beapplied through resistor 66 to the grid of pentode Tl, causing T1 toconduct more heavily and thus lowering the output voltage at point B.Conversely, were the voltage at point B to tend to fall to a lowervalue, the drop would also be fed back through resistor 66, reducing theconduction through pentode T1 and increasing the plate voltage of T1 anddecreasing the voltage between grid and cathode of pentode T2 (the gridof T2 is directly coupled to the plate of T1 The resulting increase inconduction through T2 causes the voltage at point B to rise restoringthe equilibrium output value of +2,000 volts.

The small-signal output impedance of the circuit is extremely low.Assume, for example, that pentodes T1 and T2 each have atransconductance of 1,000 microhms and that a l-volt incremental voltageis applied to point B. This incremental voltage is fed back throughresistor 66 to the grid of T1, resulting in a 1 ma. increase in theplate current of T1 and a 7-volt increase in the drop across resistor68. The resulting 7-volt increase in the grid bias of T2 reduces thecathode current of T2 by 7 ma. The total change in current at point B(as seen by the load) is thus the increase in the plate current of T1plus the decrease in the cathode current of T2 l ma. 7 ma. 8 ma). Thesmall-signal output impedance of the amplifier is therefore only Ivolt/8 ma. =.125 ohms.

Only a few tens of volts of drive are required from the plate of pentodeT3 which, in driving point A, closely approximates a very linearconstant current generator. The large unbypassed cathode resistor 62increases the effective output impedance of the T3-stage. (The effectiveresistance looking into the plate of T3 can be as high as l M.)Series-connected pentodes T1 and T2 function essentially as class Bamplifiers, but the linearity of their operation is greatly increased bythe 40 to 50 db. of negative feedback provided by resistor 66. Furtherlinearity improvement can be achieved by customary feedback from point Bto 52. i

For large signal inputs, the amplifier can furnish very high peakcurrents, both positive and negative, to-load 60. When pentode T2conducts, the output current is limited only by the current capacity ofT2 at low or zero bias, and peak currents of many hundreds of ma. can befurnished to the load.

When pentode Tl conducts, the output current is, in first instance,limited by the 7 K resistor 68. However, by shunting a lOO-volt Zenerdiode 70 across resistor 68, the voltage drop can be limited to 100volts and the current which Tl can supply to the load can then, for allpractical purposes, be limited only by the current capacity of pentodeT1 rather than by resistor 68.

A block diagram showing the entire electronic section of the system ispresented in FIG. 12. This block diagram incorporates amplifier elementssimilar to those shown in FIG. 11 and in addition shows the audiofeedback system used to damp the low frequency resonance of the system.

An audio input signal 52 is applied to input amplifier 72, ahigh-impedance, constant current stage or set of stages (which may evenbe solid state) serving the function of pentode T3 in FIG. 11, and alsoserving to combine the audio input 52 with a feedback signal on line 73.Active devices 74 and 76, which occupy roles similar to those ofpentodesTl and T2 res ectively, (but of course need not necessarily bepentodes) are connected in series between a high voltage source atterminal 56 and ground. Diaphragm 78 of electrostatic transducer 80 iselectrically connected to. Point B, the junction of active devices 74and 76, as is feedback resistor 66.

It is realized that damping of the low frequency resonance peak isdesired. To a certain degree this is possible by viscous damping, e.g.using glass wool disposed immediately adjacent to the back of thetransducer as suggested in dotted lines at 19a in FIG. 1. Electronicallythis same damping is well achieved by feedback system 82. Carriergenerator 84 applies a 260 KC signal to diaphragm 78. This signalinducescorresponding signals at electrodes 86 and 88 of electrostatictransducer 80. The amplitude of each of these signals varies with thedistance of the diaphragm from the electrode; the closer the diaphragm,the stronger the signal. The induced carrier signals on the twoelectrodes are detected and summed at diaphragm displacement detector87. Successful operation is made possible by the fact that with thedielectric electrode made as described above, a substantial K (believedto be greater than about exists at the frequency of the carrier wave.

Theresulting signal, centered around a zero voltage and ranging positiveor negative depending upon the direction of diaphragm displacement fromthe center position, is proportional to diaphragm displacement and is inquadrature with the transducer drive voltage at the resonant frequency.Diaphragm excursion should be inversely proportional to the frequencysquared (i.e. displacement decreases 12 db. for each octave of frequencyincrease). The output of diaphragm displacement detector 86 is appliedto differentiator 88, which generates a signal advanced in phase by 90(and thus approximately in phase with the drive voltage). The amplitudeof this differentiated signal is inversely proportional to frequency andso decreases 6 db. for each octave of frequency increase.

The differentiator output signal is applied to feedback amplifier whichintroduces a further 180 phase shift in the signal. The output from thefeedback amplifier is returned through line 74 to constant currentgenerator 72 and there summedwith audio input signal 52. (The feedbacksystem 82 is designed so that the line 74 feedback signal is significantonly from about 20 Hz to about 200 Hz, its phase at those frequencies issuch as to oppose the drive voltage.) To produce constant sound energythroughout the frequency range of the system (with constant driveamplitude) the system response curve should drop about 6 db. per octave.ideally approaching line in FIG. 14. The characteristics of theelectrostatic speaker are such that there would, vn'thout feedback, beabout a 15 db. resonant peak 102 in the response corresponding to theacoustic resonance of the speaker. The use of the feedback system 82 notonly flattens out this peak as shown at 104, but also renders the phaseresponse of the system far more linear. The phase response withoutfeedback is shown in FIG. 15 at 106; phase response with feedback isshown at 108.

A detailed schematic drawing is shown in FIG. 13 of those systemcomponents represented in block form in FIG. 12. This practical circuitillustrates one presently preferred implementation of the electronicportions of the electrostatic speaker system.

A further schematic drawing is shown in FIG. 16 of a system employingcertain of the-elements in solid state, and thus constituting areduction to practice using components that are practical forproduction. In this embodiment two amplifiers of identical constructionare employed, one connected to the diaphragm and the other to the fixedelectrodes as shown, with a phase shift between the two. The arrangementis generally suggested in FIG. 1 as well. For a given amount of audiosignal a considerably more powerful output is obtained, relative to aone-amplifier embodiment.

Numerous variations in the specific details are possible within thespiritand scope of the invention.

What is claimed is:

1. An electrostatic loudspeaker comprising an electrostatic transducerhaving diaphragm surface portions which produce backward and forwardmoving sound radiation, an enclosure surrounding said transducer andadapted to contain the backward moving sound, an outlet passage forforward moving-sound, said outlet passage having a throat that issubstantially less in cross-sectional area than the collective area ofsaid diaphragm surface portions, said outlet passage constructed andarranged with respect to said diaphragm surface portions to causeforward moving sound from each of said portions to pass throughsubstantially equal length paths terminating beyond said throat at aprojected common circle centered on said passage, the path for adjacentdiaphragm surface portions terminating at adjacent segments of the arcof said projected circle, and wherein said passage is provided by a setof channels defined by walls, each channel associated with apredetermined diaphragm surface portion, channels associated withneighboring surface portions having their outlets in neighboringrelation, the channels having constricted throat portions ofcross-sectional area less than the area of their respective inlets.

2. The loudspeaker of claim 1 in which said channels extend outwardlyfrom said throat portions to outlets that are larger than the respectivethroat portions.

3. An electrostatic loudspeaker comprising an electrostatic transducerhaving diaphragm surface portions which produce backward and forwardmoving sound radiation, an enclosure surrounding said transducer andadapted to contain the backward moving sound, an outlet passage forforward moving sound, said outlet passage having a throat that issubstantially less in cross-sectional area than the collective area ofsaid diaphragm surface portions, said outlet passage constructed andarranged with respect to said diaphragm surface portions to causeforward moving sound from each of said portions to pass throughsubstantially equal length paths terminating beyond said throat at aprojected common circle centered on said passage, the path for adjacentdiaphragm surface portions terminating at adjacent segments of the arcof said projected circle, and wherein said diaphragm surface portionsare arrayed within said enclosure to form substantially a segment of acircle, the inside surface of said circular array aligned with a spacedapart outlet aperture of said enclosure, said aperture beingsubstantially smaller in width than the chord of said segment, and wallsconnecting the sides of said circular array to the corresponding sidesof said aperture, whereby the paths normal to adjacent surface portionscross each other substantially at a common point and emanate radiallytherefrom to provide a circular wave front.

4. An electrostatic loudspeaker comprising an electrostatic transducerhaving diaphragm surface portions which produce backward and forwardmoving sound radiation, an enclosure surrounding said transducer andadapted to contain the backward moving sound, an outlet passage forforward moving sound, said outlet passage having a throat that issubstantially less in cross-sectional area than the collective area ofsaid diaphragm surface portions, said passage is provided by a set ofchannels defined by walls, each channel associated with a predetermineddiaphragm surface portion, channels associated with neighboring surfaceportions having their outlets in like relation, the channels havingconstricted throat portions of cross-sectional area less than the areaof their respective inlets.

5. The loudspeaker of claim 4 wherein said diaphragm portions and thecooperating outlet passage are each of elongated form in the directionperpendicular to said circle thereby adapted to generate substantially acylindrical waveform dispersion of high frequency sound.

6. The loudspeaker of claim 4 wherein said diaphragm is at least abouttwice as wide as the minimum width of said throat.

7. The loudspeaker of claim 4 wherein said diaphragm surface portionsare defined by an integral planar diaphragm, and said channels comprisemiddle channels extending substantially straight from middle portions ofsaid planar diaphragm to their respective outlets and outer channelsextending along curved paths from outer portions of the diaphragm, saidouter channels having first portions gradually reducing incross-sectional area and curving toward said middle channels and secondportions gradually increasing in cross-sectional area and curving awayfrom said middle channels toward their respective outlets.

8. The loudspeaker of claim 4 wherein the portions of said channelsimmediately adjacent said outlets have centerlines substantiallyintersecting at a point.

9. The loudspeaker of claim 8 wherein said point of intersection liessubstantially in the plane of the front of said enclosure and saidoutlets are arranged substantially through a semicircular arc.

10. An electrostatic loudspeaker capable of emitting high frequenciescomprising a diaphragm mounted adjacent at least a first electrode toform an electrostatic transducer in combination with a lens systemadapted to convert the directional high frequency sound from saiddiaphragm into a substantially curved wave, said lens system comprisingwalls defining a set of channels, each channel associated with apredetermined portion of said diaphragm, channels associated withneighboring portions having their outlets in like relation, saidchannels comprising middle channels extending substantially straightfrom middle portions of said planar diaphragm to their respectiveoutlets and outer channels extending along curved paths from outerportions of the diaphragm, first portions of said outer channels nearsaid diaphragm having gradually reducing cross-sectional area andcurving toward said middle channels and second portions of said channelshaving gradually increasing cross-sectional area and curving away fromsaid middle channels toward'the outlets of said channels for dispersingsaid high frequency sound.

11. The loudspeaker of claim 10 wherein the minimum cross-sections ofsaid channels occur at varying distances from said diaphragm therebytending to reduce detrimental effects from reflections of sound passingthrough said channels.

12. The loudspeaker of claim 10 wherein said parts of said diaphragmsurface and the entrances to the corresponding outer channels havewidths corresponding generally to the length of the shortest wave lengthdesired to be dispersed without distortion, said width being less than1% inches.

13. The loudspeaker of claim 10 wherein said channels are constructedand arranged to cause forward moving sound from each of said parts ofsaid diaphragm surface to pass through substantially equal length pathstenninating at a common circle, the outer portions of said channelslying substantially on axes that intersect, defining the center of saidcircle.

14. The loudspeaker of claim 10 wherein said diaphragm and the channelsdefined by walls are each of elongated form in the directionperpendicular to said circle, thereby adapted to generate substantiallya cylindrical wave for dispersion of high frequency sound.

15. An electrostatic loudspeaker capable of emitting low frequenciesdown to around 50 Hz comprising a wide diaphragm mounted adjacent to atleast a first electrode to form an electrostatic transducer, anenclosure surrounding said transducer and adapted to contain thebackward moving sound radiation, an extended air column defining outletpassage from said transducer adapted to conduct forward moving soundradiation out of said enclosure, said outlet passage having an entrancesized and positioned to conduct sound from a wide effective area of saiddiaphragm, said passage tapering gradually down to a throat ofcross-sectional area substantially less than said area of saiddiaphragm, said passage provided by a set of channels defined by walls,each channel associated with a predetermined diaphragm surface portion,channels associated with neighboring surface portions having theiroutlets in neighboring relation, the channels having constricted throatportions of cross-sectional area less than the area of their respectiveinlets, said enclosure sized to define an air volume exposed to the backof the diaphragm which is a plurality of times greater than the airvolume confined by said outlet passage, said enclosure and throatadapted to establish a low frequency of resonance for said loudspeaker.

16. The loudspeaker of claim 15 wherein the portion of the volume ofsaid enclosure that is exposed to the back surface of said diaphragm isat least about four times greater than the portion of the volume of saidenclosure exposed to the front surface of said diaphragm.

17. An electrostatic loudspeaker wherein two electrodes are provided,one on each side of a diaphragm mounted under self-restoring tension ina balanced transducer construction, each of said electrodes beingcomprised of a dielectric portion directed toward said diaphragm and aconductive layer disposed on the outer side of said dielectric portion,each electrode comprising a matrix of dielectric material and adispersion through said matrix of a substance having a dielectricconstant greater than about 500, each electrode having molded therein aplurality of apertures through which sound can escape from saiddiaphragm, each of said dielectric portions having a thickness greaterthan about 3 times the thickness of the air gap between the respectiveelectrode and the diaphragm in mid-position, the dielectric portionshaving a dielectric constant greater than about 30, and means forapplying a polarizing voltage between said conductive layers of saidelectrodes, and an audio amplifier connected to apply amplified audiosignals between said diaphragm and the electrodes.

18. The loudspeaker of claim 17 wherein said electrode has a volumetricresistivity in the range of l0 to 10 ohm centlmeters.

19. An electrostatic loudspeaker transducer comprising a flexiblediaphragm mounted under self-restoring tension, at least one rigidelectrode facing the diaphragm and having a conductive surface on itsopposite side, energization means for applying a polarizing voltage andan audio signal to cause corresponding vibration of the diaphragm toproduce sound, the electrode comprising a matrix of dielectric materialand a dispersion through said matrix of a substance having a dielectricconstant greater than about 500, said electrode having molded therein aplurality of apertures through which sound can escape from saiddiaphragm, said electrode having an overall dielectric constant greaterthan about 30.

20. The electrostatic loudspeaker of claim 19 wherein said electrodecontains a dispersion of another substance having a lower volumeresistivity than said matrix material, thereby reducing the volumeresistivity of said electrode below that of said matrix material.

21. The loudspeaker transducer of claim 19 including a second electrodeof like construction assembled with said first electrode and saiddiaphragm to provide a balanced construction with the diaphragm disposedbetween said electrodes, an

enclosure surrounding said transducer constructed to contain backwardmoving radiation from said diaphragm, an outlet throat from saidenclosure for sound from said diaphragm, said throat being substantiallynarrower than said diaphragm, said energization means adapted to apply avoltage greater than 2,000 volts across said electrodes.

22. The electrostatic loudspeaker of claim 15 wherein, with respect toadjacent surface portions of said diaphragm, said outlet passage isconstructed and arranged to cause forward moving sound from each of saidadjacent surface portions of said diaphragm to pass throughsubstantially equal length paths terminating beyond said throat at aprojected common circle centered on said passage, the paths for adjacentdiaphragm surface portions terminating at adjacent segments of the arcof said projected circle.

1. An electrostatic loudspeaker comprising an electrostatic transducerhaving diaphragm surface portions which produce backward and forwardmoving sound radiation, an enclosure surrounding said transducer andadapted to contain the backward moving sound, an outlet passage forforward moving sound, said outlet passage having a throat that issubstantially less in cross-sectional area than the collective area ofsaid diaphragm surface portions, said outlet passage constructed andarranged with respect to said diaphragm surface portions to causeforward moving sound from each of said portions to pass throughsubstantially equal length paths terminating beyond said throat at aprojected common circle centered on said passage, the path for adjacentdiaphragm surface portions terminating at adjacent segments of the arcof said projected circle, and wherein said passage is provided by a setof channels defined by walls, each channel associated with apredetermined diaphragm surface portion, channels associated withneighboring surface portions having their outlets in neighboringrelation, the channels having constricted throat portions ofcross-sectional area less than the area of their respective inlets. 2.The loudspeaker of claim 1 in which said channels extend outwardly fromsaid throat portions to outlets that are larger than the respectivethroat portions.
 3. An electrostatic loudspeaker comprising anelectrostatic transducer having diaphragm surface portions which producebackward and forward moving sound radiation, an enclosure surroundingsaid transducer and adapted to contain the backward moving sound, anoutlet passage for forward moving sound, said outlet passage having athroat that is substantially less in cross-sectional area than thecollective area of said diaphragm surface portions, said outlet passageconstructed and arranged with respect to said diaphragm surface portionsto cause forward moving sound from each of said portions to pass throughsubstantially equal length paths terminating beyond said throat at aprojected common circle centered on said passage, the path for adjacentdiaphragm surface portions terminating at adjacent segments of the arcof said projected circle, and wherein said diaphragm surface portionsare arrayed within said enclosure to form substantially a segment of acircle, the inside surface of said circular array aligned with a spacedapart outlet aperture of said enclosure, said aperture beingsubstantially smaller in width than the chord of said segment, and wallsconnecting the sides of said circular array to the corresponding sidesof said aperture, whereby the paths normal to adjacent surface portionscross each other substantially at a common point and emanate radiallytherefrom to provide a circular wave front.
 4. An electrostaticloudspeaker comprising an electrostatic transducer having diaphragmsurface portions which produce backward and forward moving soundradiation, an enclosure surrounding said transducer and adapted tocontain the backward moving sound, an outlet passage for forward movingsound, said outlet passage having a throat that is substantially less incross-sectional area than the collective area of said diaphragm surfaceportions, said passage is provided by a set of channels defined bywalls, each channel associated with a predetermined diaphragm surfaceportion, channels associated with neighboring surface portions havingtheir outlets in like relation, the channels having constricted throatportions of cross-sectional area less than the area of their respectiveinlets.
 5. The loudspeaker of claim 4 wherein said diaphragm portionsand the cooperating outlet passage are each of elongated form in thedirection perpendicular to said circle thereby adapted to generatesubstantially a cylindrical waveform dispersion of high frequency sound.6. The loudspeaker of claim 4 wherein sAid diaphragm is at least abouttwice as wide as the minimum width of said throat.
 7. The loudspeaker ofclaim 4 wherein said diaphragm surface portions are defined by anintegral planar diaphragm, and said channels comprise middle channelsextending substantially straight from middle portions of said planardiaphragm to their respective outlets and outer channels extending alongcurved paths from outer portions of the diaphragm, said outer channelshaving first portions gradually reducing in cross-sectional area andcurving toward said middle channels and second portions graduallyincreasing in cross-sectional area and curving away from said middlechannels toward their respective outlets.
 8. The loudspeaker of claim 4wherein the portions of said channels immediately adjacent said outletshave centerlines substantially intersecting at a point.
 9. Theloudspeaker of claim 8 wherein said point of intersection liessubstantially in the plane of the front of said enclosure and saidoutlets are arranged substantially through a semicircular arc.
 10. Anelectrostatic loudspeaker capable of emitting high frequenciescomprising a diaphragm mounted adjacent at least a first electrode toform an electrostatic transducer in combination with a lens systemadapted to convert the directional high frequency sound from saiddiaphragm into a substantially curved wave, said lens system comprisingwalls defining a set of channels, each channel associated with apredetermined portion of said diaphragm, channels associated withneighboring portions having their outlets in like relation, saidchannels comprising middle channels extending substantially straightfrom middle portions of said planar diaphragm to their respectiveoutlets and outer channels extending along curved paths from outerportions of the diaphragm, first portions of said outer channels nearsaid diaphragm having gradually reducing cross-sectional area andcurving toward said middle channels and second portions of said channelshaving gradually increasing cross-sectional area and curving away fromsaid middle channels toward the outlets of said channels for dispersingsaid high frequency sound.
 11. The loudspeaker of claim 10 wherein theminimum cross-sections of said channels occur at varying distances fromsaid diaphragm thereby tending to reduce detrimental effects fromreflections of sound passing through said channels.
 12. The loudspeakerof claim 10 wherein said parts of said diaphragm surface and theentrances to the corresponding outer channels have widths correspondinggenerally to the length of the shortest wave length desired to bedispersed without distortion, said width being less than 1 1/2 inches.13. The loudspeaker of claim 10 wherein said channels are constructedand arranged to cause forward moving sound from each of said parts ofsaid diaphragm surface to pass through substantially equal length pathsterminating at a common circle, the outer portions of said channelslying substantially on axes that intersect, defining the center of saidcircle.
 14. The loudspeaker of claim 10 wherein said diaphragm and thechannels defined by walls are each of elongated form in the directionperpendicular to said circle, thereby adapted to generate substantiallya cylindrical wave for dispersion of high frequency sound.
 15. Anelectrostatic loudspeaker capable of emitting low frequencies down toaround 50 Hz comprising a wide diaphragm mounted adjacent to at least afirst electrode to form an electrostatic transducer, an enclosuresurrounding said transducer and adapted to contain the backward movingsound radiation, an extended air column defining outlet passage fromsaid transducer adapted to conduct forward moving sound radiation out ofsaid enclosure, said outlet passage having an entrance sized andpositioned to conduct sound from a wide effective area of saiddiaphragm, said passage tapering gradually down to a throat ofcross-sectional area substantially less than saId area of saiddiaphragm, said passage provided by a set of channels defined by walls,each channel associated with a predetermined diaphragm surface portion,channels associated with neighboring surface portions having theiroutlets in neighboring relation, the channels having constricted throatportions of cross-sectional area less than the area of their respectiveinlets, said enclosure sized to define an air volume exposed to the backof the diaphragm which is a plurality of times greater than the airvolume confined by said outlet passage, said enclosure and throatadapted to establish a low frequency of resonance for said loudspeaker.16. The loudspeaker of claim 15 wherein the portion of the volume ofsaid enclosure that is exposed to the back surface of said diaphragm isat least about four times greater than the portion of the volume of saidenclosure exposed to the front surface of said diaphragm.
 17. Anelectrostatic loudspeaker wherein two electrodes are provided, one oneach side of a diaphragm mounted under self-restoring tension in abalanced transducer construction, each of said electrodes beingcomprised of a dielectric portion directed toward said diaphragm and aconductive layer disposed on the outer side of said dielectric portion,each electrode comprising a matrix of dielectric material and adispersion through said matrix of a substance having a dielectricconstant greater than about 500, each electrode having molded therein aplurality of apertures through which sound can escape from saiddiaphragm, each of said dielectric portions having a thickness greaterthan about 3 times the thickness of the air gap between the respectiveelectrode and the diaphragm in mid-position, the dielectric portionshaving a dielectric constant greater than about 30, and means forapplying a polarizing voltage between said conductive layers of saidelectrodes, and an audio amplifier connected to apply amplified audiosignals between said diaphragm and the electrodes.
 18. The loudspeakerof claim 17 wherein said electrode has a volumetric resistivity in therange of 108 to 1011 ohm centimeters.
 19. An electrostatic loudspeakertransducer comprising a flexible diaphragm mounted under self-restoringtension, at least one rigid electrode facing the diaphragm and having aconductive surface on its opposite side, energization means for applyinga polarizing voltage and an audio signal to cause correspondingvibration of the diaphragm to produce sound, the electrode comprising amatrix of dielectric material and a dispersion through said matrix of asubstance having a dielectric constant greater than about 500, saidelectrode having molded therein a plurality of apertures through whichsound can escape from said diaphragm, said electrode having an overalldielectric constant greater than about
 30. 20. The electrostaticloudspeaker of claim 19 wherein said electrode contains a dispersion ofanother substance having a lower volume resistivity than said matrixmaterial, thereby reducing the volume resistivity of said electrodebelow that of said matrix material.
 21. The loudspeaker transducer ofclaim 19 including a second electrode of like construction assembledwith said first electrode and said diaphragm to provide a balancedconstruction with the diaphragm disposed between said electrodes, anenclosure surrounding said transducer constructed to contain backwardmoving radiation from said diaphragm, an outlet throat from saidenclosure for sound from said diaphragm, said throat being substantiallynarrower than said diaphragm, said energization means adapted to apply avoltage greater than 2,000 volts across said electrodes.
 22. Theelectrostatic loudspeaker of claim 15 wherein, with respect to adjacentsurface portions of said diaphragm, said outlet passage is constructedand arranged to cause forward moving sound from each of said adjacentsurface portions of said diaphragm to pass Through substantially equallength paths terminating beyond said throat at a projected common circlecentered on said passage, the paths for adjacent diaphragm surfaceportions terminating at adjacent segments of the arc of said projectedcircle.